Tissue, such as a benign or malignant tumor, within a skull or other region of a body may be treated invasively, e.g., by surgically removing the tissue, or non-invasively, e.g., using thermal ablation. Both approaches may effectively treat certain localized conditions within the brain, but involve delicate procedures in which it is desired to avoid destroying or damaging otherwise healthy tissue. These treatments may not be appropriate for conditions in which diseased tissue is integrated into healthy tissue, unless destroying the healthy tissue is unlikely to affect neurological function significantly.
Thermal ablation, as may be accomplished using focused ultrasound, has particular appeal for treating tissue within the brain and other tissue regions deep within the body, because it generally does not disturb intervening or surrounding healthy tissue. Focused ultrasound may also be attractive, because acoustic energy generally penetrates well through soft tissues, and ultrasonic energy, in particular, may be focused towards focal zones having a cross-section of only a few millimeters due to relatively short wavelengths (e.g., as small as 1.5 millimeters (mm) in cross-section at one Megahertz (1 MHz)). Thus, ultrasonic energy may be focused at a region deep within the body, such as a cancerous tumor or other diseased tissue, to ablate the diseased tissue without significantly damaging surrounding healthy tissue.
To focus ultrasonic energy towards a desired target, a piezoelectric transducer may be used that includes a plurality of transducer elements. A controller may provide drive signals to each of the transducer elements, thereby causing the transducer elements to transmit acoustic energy such that constructive interference occurs at a “focal zone.” At the focal zone, sufficient acoustic energy may be delivered to heat tissue within the focal zone until tissue necrosis occurs, i.e., until the tissue is destroyed. Preferably, tissue along the path through which the acoustic energy passes (“the pass zone”) outside the focal zone, is heated only minimally, if at all, thereby minimizing damaging tissue outside the focal zone.
As acoustic energy passes through tissue, the acoustic energy may interact with the tissue through multiple processes: propagation, scattering, absorption, reflection, and refraction. The intensity of the acoustic energy transmitted by the transducer array generally determines the therapeutic effectiveness, i.e., the volume of tissue destroyed within the focal zone (although there may be some losses as the acoustic energy interacts with intervening tissue between the transducer and the focal zone). The size of the focus zone may also depend upon system parameters, such as transducer element characteristics, frequency of the acoustic energy, and focal depth (the distance from the transducer to the focal zone), as well as patient-related parameters, such as tissue inhomogeneity.
When a transducer is activated, the relative phase of drive signals delivered to each transducer element may be adjusted based upon the distance of the respective transducer element from the focal zone. Generally, an average speed of sound is used to approximate the speed at which the acoustic energy passes through tissue, e.g., 1540 meters per second (m/s), and to predict the location of the focal zone.
While system parameters are generally fixed for a given transducer array, tissue homogeneity may vary significantly from patient to patient, and even between different tissue regions within the same patient. Tissue inhomogeneity may decrease intensity of the acoustic energy at the focal zone and may even move the location of the focal zone within the patient's body. Specifically, because the speed of sound differs in different types of tissue, as portions of a beam of acoustic energy travel along different paths towards the focal zone, they may experience a relative phase shift or time delay, which may change the intensity at the focal zone and/or move the location of the focal zone.
For example, the speed of sound through fat is approximately 1460 meters per second (m/s), while the speed of sound through muscle is approximately 1600 meters per second (m/s). The speed of sound through bone tissue is much faster, for example, approximately 3000 meters per second (m/s) for skull bone tissue. The speed of sound also varies in different organs. For example, the speed of sound in brain tissue is approximately 1570 meters per second (m/s), approximately 1555 meters per second (m/s) in the liver, and approximately 1565 meters per second (m/s) in the kidney.
Since a beam of acoustic energy has a relatively wide aperture where it enters the body, different parts of the acoustic energy may pass through different tissue pass zones, and therefore may pass through different tissue types. Thus, when acoustic energy is transmitted through tissue, portions of the acoustic energy may experience different speeds of sound, which may shift the relative phases of acoustic energy transmitted from respective transducer elements. This phase shifting may decrease the constructive interference of the acoustic energy at the focal zone, which may reduce the effectiveness of the treatment, or may even move the focal zone in an unpredictable manner. For example, a layer of fat that is only seven millimeters (7 mm) thick within muscle tissue may introduce a phase shift of 180° at an ultrasonic frequency of one Megahertz (1 MHz), which would change desired constructive interference at the focal zone into destructive interference.
Tissue inhomogeneity may also cause refraction of acoustic energy at the boundaries of tissue regions having different speeds of sound. Refraction may decrease constructive interference, and hence, the intensity of the acoustic energy at the focal zone, particularly when the acoustic energy passes through bone. Thus, inhomogeneous tissue structures may generate beam aberrations and refractions, which may distort the focus and reduce the intensity, thus affecting treatment efficiency.
Accordingly, systems and methods for effectively focusing acoustic energy towards a desired focal zone would be useful.